Code dojo solution for Week 4 with @sofiajeanne

project-euler-12.hs

import Control.Monad.Trans.State
import Data.Map.Strict as Map
import Data.Set as Set

triangle n = sum [1..n]
factors n = [f | f <- [1..n], n `mod` f == 0 ]

addFactors :: Int -> Int -> Map.Map Int (Set.Set Int) -> Map.Map Int (Set.Set Int)
addFactors n i factorMap 
 | n == i = Map.insert n (Set.fromList (factors n)) factorMap
 | otherwise = case () of
     _ | Map.member n factorMap -> factorMap
     _ | n `rem` i /= 0         -> addFactors n (i+1) factorMap
     _ | n `rem` i == 0         -> let
         q           = n `div` i
         factorMap'  = addFactors i 2 factorMap
         factorMap'' = addFactors q 2 factorMap'
         qFactors    = factorMap'' Map.! q
         qMultiples  = Set.map (*i) qFactors
         nFactors    = Set.union qMultiples qFactors
         in Map.insert n nFactors factorMap''

factor1 = Map.singleton 1 $ Set.singleton 1
factorCount n = do
    knownFactors <- get
    let updated = addFactors n 2 knownFactors
    put updated
    return $ Set.size $ updated Map.! n

main = print $ triangle . fst . head $ Prelude.filter ((>500) . snd) $
    zip [10000..20000] $ evalState (mapM (factorCount . triangle) [10000..20000]) factor1